Estrogen prevents osteoporotic bone loss by attenuating bone resorption; however, the molecular basis for this is unknown. Here, we report a critical role for the osteoclastic estrogen receptor alpha (ERalpha) in mediating estrogen-dependent bone maintenance in female mice. We selectively ablated ERalpha in differentiated osteoclasts (ERalpha(DeltaOc/DeltaOc)) and found that ERalpha(DeltaOc/DeltaOc) females, but not males, exhibited trabecular bone loss, similar to the osteoporotic bone phenotype in postmenopausal women. Further, we show that estrogen induced apoptosis and upregulation of Fas ligand (FasL) expression in osteoclasts of the trabecular bones of WT but not ERalpha(DeltaOc/DeltaOc) mice. The expression of ERalpha was also required for the induction of apoptosis by tamoxifen and estrogen in cultured osteoclasts. Our results support a model in which estrogen regulates the life span of mature osteoclasts via the induction of the Fas/FasL system, thereby providing an explanation for the osteoprotective function of estrogen as well as SERMs.

The liver and exocrine pancreas share a common structure, with functioning units (hepatic plates and pancreatic acini) connected to the ductal tree. Here we show that Sox9 is expressed throughout the biliary and pancreatic ductal epithelia, which are connected to the intestinal stem-cell zone. Cre-based lineage tracing showed that adult intestinal cells, hepatocytes and pancreatic acinar cells are supplied physiologically from Sox9-expressing progenitors. Combination of lineage analysis and hepatic injury experiments showed involvement of Sox9-positive precursors in liver regeneration. Embryonic pancreatic Sox9-expressing cells differentiate into all types of mature cells, but their capacity for endocrine differentiation diminishes shortly after birth, when endocrine cells detach from the epithelial lining of the ducts and form the islets of Langerhans. We observed a developmental switch in the hepatic progenitor cell type from Sox9-negative to Sox9-positive progenitors as the biliary tree develops. These results suggest interdependence between the structure and homeostasis of endodermal organs, with Sox9 expression being linked to progenitor status.

Chondrogenesis is a multistep process that is essential for endochondral bone formation. Previous results have indicated a role for ␤-catenin and Wnt signaling in this pathway. Here we show the existence of physical and functional interactions between ␤-catenin and Sox9, a transcription factor that is required in successive steps of chondrogenesis. In vivo, either overexpression of Sox9 or inactivation of ␤-catenin in chondrocytes of mouse embryos produces a similar phenotype of dwarfism with decreased chondrocyte proliferation, delayed hypertrophic chondrocyte differentiation, and endochondral bone formation. Furthermore, either inactivation of Sox9 or stabilization of ␤-catenin in chondrocytes also produces a similar phenotype of severe chondrodysplasia. Sox9 markedly inhibits activation of ␤-catenin-dependent promoters and stimulates degradation of ␤-catenin by the ubiquitination/proteasome pathway. Likewise, Sox9 inhibits ␤-catenin-mediated secondary axis induction in Xenopus embryos. ␤-Catenin physically interacts through its Armadillo repeats with the C-terminal transactivation domain of Sox9. We hypothesize that the inhibitory activity of Sox9 is caused by its ability to compete with Tcf/Lef for binding to ␤-catenin, followed by degradation of ␤-catenin. Our results strongly suggest that chondrogenesis is controlled by interactions between Sox9 and the Wnt/␤-catenin signaling pathway. Chondrogenesis, an obligatory process in endochondral bone formation, starts with the recruitment of chondrogenic mesenchymal cells into condensations. This is followed by the differentiation of these cells into chondrocytes, which produce cartilage-specific extracellular matrix (ECM) proteins including type II collagen and the proteoglycan aggrecan. Chondrocytes then undergo a unidirectional proliferation to form orderly parallel columns, exit the cell cycle, become prehypertrophic, and then hypertrophic. Sox9, a high-mobility-group (HMGbox) transcription factor, is required at sequential steps in this pathway (Bi et al. 1999(Bi et al. , 2001Akiyama et al. 2002).Both the human disease campomelic dysplasia, which is caused by heterozygous mutations in the Sox9 gene and is due to Sox9 haploinsufficiency, as well as Sox9 heterozygous mutant mice are characterized by a general hypoplasia of endochondral bones (Foster et al. 1994;Wagner et al. 1994). Inactivation of Sox9 in limb buds using the Cre recombinase/loxP recombination system before chondrogenic mesenchymal condensations results in the complete absence of mesenchymal condensations and of subsequent cartilage and bone formation, indicating that Sox9 is needed for an early step in chondrogenesis, that of mesenchymal condensations (Akiyama et al. 2002). A similar conclusion was also reached by analysis of mouse embryo chimeras derived from homozygous Sox9 mutant embryonic stem (ES) cells (Bi et al. 1999). That Sox9 is needed at sequential steps is shown by the severe generalized chondrodysplasia of mouse embryos in which Sox9 is deleted after chondrogenic mesenchymal conde...

Objective. Stromal cell-derived factor 1 (SDF-1; CXCL12/pre-B cell growth-stimulating factor) is a dominant chemokine in bone marrow and is known to be involved in inflammatory diseases, including rheumatoid arthritis. However, its role in bone repair remains unknown. The purpose of this study was to investigate the role of SDF-1 and its receptor, CXCR4, in bone healing.Methods. The expression of SDF-1 during the repair of a murine structural femoral bone graft was examined by real-time polymerase chain reaction and immunohistochemical analysis. The bone graft model was treated with anti-SDF-1 neutralizing antibody or TF14016, an antagonist for CXCR4, and evaluated by histomorphometry. The functional effect of SDF-1 on primary mesenchymal stem cells was determined by in vitro and in vivo migration assays. New bone formation in an exchanging-graft model was compared with that in the autograft models, using mice partially lacking SDF-1 (SDF-1 ؉/؊ ) or CXCR4 (CXCR4 ؉/؊ ).Results. The expression of SDF1 messenger RNA was increased during the healing of live bone grafts but was not increased in dead grafts. High expression of SDF-1 protein was observed in the periosteum of the live graft. New bone formation was inhibited by the administration of anti-SDF-1 antibody or TF14016. SDF-1 increased mesenchymal stem cell chemotaxis in vitro in a dose-dependent manner. The in vivo migration study demonstrated that mesenchymal stem cells recruited by SDF-1 participate in endochondral bone repair. Bone formation was decreased in SDF-1 ؉/؊ and CXCR4 ؉/؊ mice and was restored by the graft bones from CXCR4 ؉/؊ mice transplanted into the SDF-1 ؉/؊ femur, but not vice versa.Conclusion. SDF-1 is induced in the periosteum of injured bone and promotes endochondral bone repair by recruiting mesenchymal stem cells to the site of injury.